Work. Word used, followed by a number, e.g. Opus 50, for the numbering of a composer's works. This numbering gives a rough idea of the order in which works were comp., but can be misleading. Sometimes the Opus no. is allotted by the composer, sometimes by the publisher. Some composers, e.g. Mozart, Haydn, did not number their works; some, e.g. Elgar, gave some works opus nos. and not others; some, e.g. R. Strauss, did likewise but also reallotted opus nos. so that much confusion arises in his case. Dvořák allowed early works to be given late opus nos. by his publisher. In many cases an opus no. covers a group of works, in which case the numbering is subdivided, e.g. Op.59, No.3, or in a style often used, Op.59/3. In other cases, 2 versions of the same work exist and the composer uses letters after the number to differentiate them, e.g. Op.49a, Op.49b. Although the Latin plural of opus is opera, it has become customary to write ‘opuses’, to avoid confusion, just as in Italian ‘opera’ has become a singular noun with the plural opere.